A theory to explain the fertility-intelligence relationship is that while income and IQ are positively correlated, income is also in itself a fertility factor that correlates inversely with fertility, that is, the higher the incomes, the lower the fertility rates and vice versa. There is thus an inverse correlation between income and fertility within and between nations. The higher the level of education and GDP per capita of a human population, sub-population or social stratum, the fewer children are born. In a 1974 UN population conference in Bucharest, Karan Singh, a former minister of population in India, encapsulated this relationship by stating "Development is the best contraceptive".

Typically IQ, access to education, and income are conversely related to number of children. Its actually most predominant in America amongst black and brown people, whose most brilliant have next to no children.